(1) Field of the Invention
In metal forming processes, metal melt is transferred from one metallurgical vessel to another or to a mould. For example a ladle is filled with metal melt out of a furnace and transferred to a tundish. The metal melt can then be cast from the tundish to a mould for forming slabs, billets or blooms. Alternately, ingots could also be poured into direct from the ladle. In most cases, it is desirable to blow a gas into the molten metal contained in such metallurgical vessels. This can be useful to accelerate the homogenization of the temperature and composition of a bath, to carry non-metallic inclusions present in the bulk of the bath up into the slag top layer, to create favourable conditions within the molten metal, and the like. The gas is generally blown into the molten metal by means of refractory purging devices such as purging plugs which are located at the bottom or side of a metallurgical vessel such as a ladle or a tundish.
(2) Description of the Related Art
Refractory purging devices such as purging plugs usually comprises a block of refractory material, generally extending along a longitudinal axis. At one longitudinal end of the block, a gas inlet connected to a source of pressurized gas is fluidly connected to a gas outlet at the opposite longitudinal end of the block. The gas inlet and gas outlet may be fluidly connected to one another through an open pore network, by one or more channels (e.g., slit shaped or with circular cross-section), or a combination of both. An open pore network is sometimes said to yield “indirect permeability,” whilst a channel is said to yield “direct permeability.” It is generally recognized that indirect permeability plugs are more efficient than direct permeability plugs, in terms of opening rate and stir effectiveness (due to bubble size and working area). One downside to porous plugs (indirect) is that the material usually is weaker (less hot and cold crushing strength and faster erosion at higher flow rates). Examples of refractory gas purging devices are described in U.S. Pat. No. 5,478,053, U.S. Pat. No. 5,820,816 and U.S. Pat. No. 6,669,896.
One known problem with refractory purging devices such as gas purging plugs is that in case of a reduction of pressure when the flow of pressurised air is stopped, molten metal may flow reversely into the device and infiltrate the channels and/or pores of the device through the gas outlet, driven by gravity. This creates not only a security concern but also an operational problem. Indeed, upon freezing of the molten melt in the channels or pores, the device is at least partly clogged. It is possible to unclog a clogged device by blowing high pressure gas into the channels and/or pores or by oxygen lancing the upper surface of the plug when the metallurgical vessel is empty, but these techniques are time consuming, are not always suitable, lead to severe erosion of the refractory material and do not always succeed to unclog the plug. Unless a plurality of independent channels are used, the infiltration by molten metal at one point of the gas outlet, usually leads to the complete or partial clogging of the whole device. To lower the risk of infiltration, the diameter or width of a channel is generally limited to not more than 1 mm, in order to offer a sufficient resistance to infiltration by capillarity. This measure is, however, detrimental to the efficacy of the purging device, as it strongly limits the gas flow rate through the plug.